Transparent conductive films have been widely used in various applications, for example, as film electrodes in radio frequency identification tags, display devices, lighting devices, electronic paper, solar cells, transistors, integrated circuits, lasers, sensors, and so on. Currently, the most popular materials for preparing transparent conductive films are metal oxides, such as indium tin oxide (ITO), indium zinc oxide (IZO) and the like. However, such metal oxides are brittle and also expensive due to high content of rare metals, resulting in that the applications thereof are greatly limited.
Apart from the above-said brittle films made of metal oxides, there is another kind of flexible transparent conductive films in the art. The flexible transparent conductive films have broader applications because they are flexible and bendable, as compared with metal oxide conductive films. Particularly, the flexible transparent conductive films are suitable for use in flexible organic electroluminescent devices (OLEDs). The materials for making the flexible transparent conductive films known in the art are mainly conductive polymers and carbon nanotubes. Traditional conductive polymers including, for example, polyaniline (PANI) and polypyrrole (PPy), are poor in conductivity. So, other conductive polymers have been developed, including, for example, poly(3,4-ethylenedioxythiophene monomer:polystyrene sulfonate) (i.e., PEDOT:PSS), which have good conductivity but are expensive. The carbon nanotubes have excellent photoelectric and mechanical properties. However, techniques of preparing and purifying the carbon nanotubes are complex and it is difficult to separate semi-conductive carbon nanotubes from the metallic carbon nanotubes, resulting in that they are hardly industrially used on a large scale.
Graphene is a two-dimensional sheet comprising a monolayer of graphite, wherein carbon atoms are in sp2 hybridization mode (like a benzene ring) and form a honeycomb-like hexagonal lattice, as shown in FIGS. 1 and 2. The carbon atoms 1 at the edge of the graphene sheet may adsorb other atoms 2 (e.g., hydrogen atom) to satisfy its valence. With such a structure, graphene is not only very stable, but also flexible upon exposure to an applied force because the bonds between the carbon atoms thereof are flexible. It has been found that graphene is not only stretchable like a rubber, but also stronger than a diamond. Furthermore, graphene exhibits an electrical conductivity comparable to copper. The electrical conductivity of graphene is so stable that it is not affected even when being stretched (or bent) by a ratio of over 40%. In addition, graphene products of large size can be produced practically and cost-effectively by the well-established techniques such as chemical vapor deposition, chemical reduction and the like.
Due to the excellent properties as described above, graphene is a promising material for flexible transparent conductive films. However, some defects have been found in the existing graphene materials. First of all, the evaporation temperatures of the graphene materials are relatively high (about 400° C.), such that it is difficult to control the evaporation process to obtain a graphene film with a uniform thickness (currently, the film is often formed by vapor deposition). In addition, the graphene materials have a relatively low work function of about 4.4 eV (the term “work function” refers to the minimum energy required to move an electron inside an object to its surface, indicating its ability to bind up electrons). In contrast, flexible OLEDs usually require the organic substances used therein to have a work function of about 5.2 eV, which is substantially higher than the work function of the graphene materials. Such a mismatch makes hole-injection difficult to take place and causes the luminescence efficiency of the OLEDs to decrease.